The present invention relates to thickness and refractive index monitoring and control systems for thin film deposition. In particular, the present invention relates to methods and apparatus for in-situ monitoring and control of deposition rate, film thickness and refractive index of thin films.
Many optical elements require optical thin film coatings. Optical coatings are deposited by a variety of techniques including evaporation, magnetron sputtering, and ion beam deposition. There are many applications that require deposition of thin films with precisely controlled thickness and refractive index over the active area of the element. Typically numerous substrates, each including many elements, are processed simultaneously. Thus, highly uniform deposition is required over wide areas for many applications.
For example, optical filters for applications such as optical fiber communication systems may require multiple layers of highly uniform thin films, where each layer has a precise thickness. Optical fiber communication systems are now widely deployed. Recently new communications services such as the Internet, high-speed data links, video services, and wireless services have resulted in a dramatic increase in the need for bandwidth. Data traffic is currently increasing at a rate of 80% per year and voice traffic is currently increasing at a rate of 10% per year.
One way of increasing bandwidth in optical fiber communications system is to increase the number of wavelengths of light propagating in the optical fiber. Wavelength division multiplexing (WDM) is an optical technology that propagates many wavelengths in the same optical fiber, thus effectively increasing the aggregate bandwidth per fiber to the sum of the bit rates of each wavelength. Bandwidths greater than 1 terabits/sec have been demonstrated in WDM based communication systems.
Dense Wavelength Division Multiplexing (DWDM) is a technology that implements WDM technology with a large number of wavelengths. DWDM is typically used to describe WDM technology that propagates more than 40 wavelengths in a single optical fiber. As the number of wavelengths increases, the channel width and channel spacing decreases. To achieve the required channel width and channel spacing in DWDM communication systems, high quality, high performance optical filters are required. These optical filters must exhibit low loss and narrow band transmission characteristics over the wavelength spectrum of 1.3 xcexcm to 1.62 xcexcm with good mechanical properties, which are stable in typically operating environments.
For example, DWDM communication systems require many band-pass filters that can separate a single wavelength (channel) from the other wavelengths (channels) propagating in the system. One type of optical filter that is used as a bandpass filter in DWDM communication systems is a Fabry Perot interference filter. Fabry Perot filters comprise two high-reflectance multi-layers separated by a xcex/2 layer. In operation, multiple interferences in the xcex/2 space layer cause the filter output spectral characteristic to peak sharply over a narrow band of wavelengths that are multiples of the xcex/2 space layer.
Another type of optical filter used in DWDM communication systems is a dielectric thin film interference filter. These filters comprise alternative layers of high refractive index and low refractive index material. Each layer is a xcex/4 thick. In operation, light reflected from high index layers does not experience a phase shift, but light reflected from the low index layers does experiences a 180 degree phase shift. Successive reflections recombine constructively at the front face producing a highly reflected beam having a narrow wavelengths range. Light having wavelengths outside of this narrow range is reflected at only very low intensity levels.
A dielectric thin film interference filter can be fabricated by depositing alternating layers of high and low refractive index material onto a glass substrate. For example, alternating layers of SiO2 and Ta2O5 can be used. The refractive index and the uniformity across the filter must be controlled to a very high precision in order to achieve the desired filter characteristics.
There is presently a need for deposition systems that can deposit optical thin films with precisely controlled thickness and refractive index. Furthermore, there is a growing need for such system with the capability of depositing these optical thin films in high volume.
The present invention relates to in-situ monitoring and control of at least one of thickness and refractive index of optical thin films during deposition. In one embodiment, the present invention determines thickness and refractive index of an optical thin film in-situ from measurements of the transmission of multi-wavelength light through the optical thin film. In another embodiment, the optical thin film thickness and refractive index can be determined independent of environmental effects and of instabilities in the optical source and optical receiver by using an algorithm that normalizes the transmission of light through the optical thin film.
Accordingly, in one embodiment, the present invention features a method of determining at least one of the thickness and refractive index of an optical thin film. The method includes generating a diagnostic light beam having a first and a second wavelength. The method also includes measuring unattenuated light intensities at the first and the second wavelength of the diagnostic light beam. The method also includes measuring attenuated light intensities at the first and the second wavelength of the diagnostic light beam after transmission through an optical thin film. The method further includes measuring null light intensities at the first and the second wavelength of the diagnostic light beam after transmission through a substantially opaque material. A first and second normalized intensity function is determined using the measured unattenuated light intensities, the measured attenuated light intensities, and the measured null light intensities. At least one of the thickness and refractive index of the optical thin film is then determined by solving the first and second normalized intensity function for thickness and refractive index.
In another embodiment, the method includes solving the first and second normalized intensity function for at least one of the absolute thickness and refractive index of the optical thin film. In another embodiment, solving for at least one of the absolute thickness and refractive index of an optical thin film is performed in-situ.
The method further includes measuring the unattenuated light intensities of the diagnostic light beam by detecting the intensities of the diagnostic light beam after transmission through a substantially transparent material.
In another aspect, the method includes determining the first normalized intensity function by determining a ratio of the difference of the measured attenuated light intensity at the first wavelength and the measured null light intensity at the first wavelength to the difference of the measured unattenuated light intensity at the first wavelength and the measured null light intensity at the first wavelength. The method also includes determining the second normalized intensity function by using a ratio of the difference of the measured attenuated light intensity at the second wavelength and the measured null light intensity at the second wavelength to the difference of the measured unattenuated light intensity at the second wavelength and the measured null light intensity at the second wavelength.
In another embodiment, at least two of the measured unattenuated light intensities, the measured attenuated light intensities, and the measured null light intensities are performed substantially simultaneously in time.
In another embodiment, the invention features an apparatus for determining at least one of thickness and refractive index of an optical thin film. The apparatus includes an optical source that generates a diagnostic light beam at a first and a second wavelength. The apparatus also includes a substrate support including a first, second, and third region. Each of the first, second, and third regions are positioned to receive the diagnostic light beam and to attenuate the diagnostic light beam with a different attenuation. One of the first, second, and third regions is adapted to support a substrate having an optical film. The apparatus further includes at least one detector positioned to receive the diagnostic light beam after transmission through each of the first, second, and third regions. The at least one detector is adapted to measure the light intensity of the diagnostic light beam after transmission through the first, second, and third regions. A processor determines at least one of the thickness and refractive index of the optical thin film from a first and a second normalized intensity function. The first and the second normalized intensity function is determined from the measured light intensity of the diagnostic light beam after transmission through the first, second, and third regions.
In one embodiment, the at least one detector is a single detector. In another embodiment, the at least one detector is a plurality of detectors. In still another embodiment, the at least one detector includes a first, a second, and a third detector that each detect light intensity of the diagnostic light beam after transmission through the first, second, and third regions, respectively.
In one embodiment, the substrate support is adapted to support a plurality of substrates. In another embodiment, at least one of the first, second, and third regions includes a substantially transparent region. In still another embodiment, at least one of the first, second, and third regions includes a substantially opaque region.
In yet another embodiment, the optical source includes a laser and the detector includes a photodetector. In another embodiment, the laser is a tunable multi-wavelength laser. In still another embodiment, the apparatus includes a lock-in amplifier electrically coupled to the detector and the laser for synchronizing electrical signals received from the detector with a rotation rate of the substrate support. Another embodiment includes a bandpass filter electrically coupled to the detector.
In another embodiment, the present invention features a method of determining at least one of the thickness and refractive index of an optical thin film. The method includes generating a diagnostic light beam having a first and a second wavelength. The method also includes measuring light intensities at the first and the second wavelength of the diagnostic light beam. The method also includes measuring light intensities at the first and the second wavelength of the diagnostic light beam after transmission through an optical thin film. The method further includes measuring null light intensities at the first and the second wavelength of the diagnostic light beam. A first and second normalized intensity function is determined using the measured light intensities of the diagnostic light beams at the first and the second wavelength, the measured light intensities of the diagnostic light beams at the first and the second wavelength, and the measured null light intensities at the first and the second wavelength. At least one of the thickness and refractive index of the optical thin film is then determined by solving the first and second normalized intensity function for at least one of thickness and refractive index.